On 2017-05-24 12:03, Mark Waddingham via use-livecode wrote:
Perhaps a better model would be to use 'continue', rather than 'break':
switch yourName
case "Mark"
-- do something and finish
case "John"
continue
case "Jon"
-- do something else
end switch
The idea being that 'continue' in a case, jumps to the next choice to
check. This works particularly nicely when you don't have an
expression, so the cases can be expressions as that essentially gives
you a 'match' type construct with back-tracking (quite useful for
processing hierarchical arrays and transforming them - a common
operation in writing compilers, for example).
Okay, so that isn't quite right - for a switch-on-value switch, continue
would have to continue into the top the next case, ignoring the case
value which is not quite the same as it would be in switch-on-case-expr
switch.
Certainly a 'fallthrough' type keyword would work... However, stepping
back and looking at the utility of switch. I'd say that in almost all
cases you either want to:
1) Map a single case to a block of code
2) Map multiple cases to a block of code
The use-case which causes subtle bugs and errors is where one case
requires a 'bit more code' before the others (where code, fallthrough,
code is *not* erroneous but intended) - also it is a rarely used pattern
in C (because it is difficult to maintain).
Taking this into account, then perhaps a better solution would be
('choose' used here for the sake of argument):
choose tValue
when 1
when 2
when 3
-- executes if tValue is 1, 2 or 3
-- never falls through
when 4
-- never falls through
default
end choose
This caters for both cases (1) and (2) and is unambiguous.
As I said before, the lack of switch in LCB isn't because we don't want
a construct like that; just that we'd like one which doesn't have the
issues of C-style switch. (The best way to avoid people accidentally
introducing subtle bugs, is to ensure the language is designed to not
let them do so!).
Warmest Regards,
Mark.
--
Mark Waddingham ~ m...@livecode.com ~ http://www.livecode.com/
LiveCode: Everyone can create apps
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